This invention relates to a security feature for safeguarding value documents, a value document, in particular a bank note, having such a security feature, a method for checking the authenticity of the value document by analyzing single particles of the security feature by spatially resolved microscopic methods (hereinafter also designated as “forensic methods”) and a use of the security feature for safeguarding a value document.
The safeguarding of value documents against forgery by means of security features has been known for some time. Feature substances are known that are based e.g. on magnetic, thermal, electrical and/or optical (e.g. absorption and emission) effects, through which the specific detectability is guaranteed.
DE 10 2012 013 244 A1 describes a value document having luminescing, particulate agglomerates which each contain at least two different luminescing, solid homogeneous phases emitting at first and second emission wavelengths, respectively. By means of the agglomeration of two luminescent pigments in one particle there is effectuated a correlation of their luminescence intensities at different measuring sites of the value document. The method for checking the presence of a value document comprises, more precisely, the following steps:    a) exciting the luminescing substances to emission;    b) capturing measurement values in a macroscopically location-resolved and wavelength-selective manner for the radiation emitted by the luminescing substances, there being produced for each of the first and the second emission wavelength measurement-value pairs involving emission wavelength and location, to thereby obtain first luminescence intensities at the first emission wavelength and second luminescence intensities at the second emission wavelength;    c) checking whether a statistical correlation is present between the first luminescence intensities and the second luminescence intensities.However, this print does not describe forensic security features (i.e. security features whose authentication is based on the analysis of single particles by spatially resolved microscopic methods) in which the substructure of individual feature substance particles is detected and identified in a spatially resolved manner by means of spatially resolved analytical techniques (such as spatially resolved X-ray diffraction, spatially resolved Raman scattering, energy-dispersive X-ray spectroscopy or spatially resolved X-ray fluorescence analysis).
US 2012/0175528 A1 describes a particulate composition having particles of which at least one particle contains at least two different crystalline phases and/or glass phases. Each phase comprises a host lattice and a dopant sensitive to electromagnetic radiation. The luminescence emission of the individual luminescing phases yields a well-defined mixed signal as a result of the agglomeration. However, the relatively homogeneously constructed agglomerates described in this prior art are unsuitable as a forensic security feature. The agglomerates are manufactured by a special spray-drying process. The individual phase regions of the different luminescent pigments in the agglomerate are hereby very small, i.e. sometimes only a few nanometers in size, with the total particle size amounting to <10 μm. The agglomerates as a whole thereby appear to be constructed homogeneously, so to speak, which is advantageous for use as a mixed-signal emitting luminescent pigment as intended in the print US 2012/0175528 A1. Upon use as a forensic security feature as intended by the present application, however, one can no longer detect any spatially clearly separate regions for the two kinds of luminescent pigment. The agglomerate can therefore not, or not readily, be distinguished from an individual luminescent pigment emitting a luminescence signal that is identical or similar to the mixed signal.
WO 2009/071167 A2 describes a forensic marking of paints, plastics, etc., with a security pigment consisting of colorant particles embedded in an inorganic transparent matrix. For detecting the forensic marker, first the outer shape and size of the security pigment and then the color or shape, size and number of the embedded colorant particles are determined under the microscope. Here, the wide availability of microscopes as the aid used for detection has an adverse effect on the security of the marking, since the essential aspects of the marking can be easily recognized in the case of a forgery attack.
US 2003/0132538 A1 describes the encapsulating of two or more fluorescence carriers for different applications. There are furthermore described a plurality of applicable analytical methods, including a “luminometer microscope”. However, no security features are described in this print.
The print WO 2010/048535 A1 describes a VIS-VIS feature system based on a luminophore mixture in which the two mixture components can be distinguished by their specific excitation spectrum. However, neither are agglomerates used, nor forensic analyses at the single-particle level carried out.
The print WO 2012/094108 A1 describes porous polymer particles for toners or security applications, which are loaded in the individual pores with a plurality of distinguishable, in particular fluorescent, radioactive or IR-absorbent markers. The pore size amounts to 20 nm to 4 μm, the particle size 2 μm to 75 μm. However, no forensic analyses at the single-particle level are described.
The print WO 2010/138914 A1 describes the combination of SERS nanoparticles with luminescent substances, magnetic substances and substances with a distinguishable mass spectrum or XRF spectrum, wherein the combination is effectuated via an encapsulation, e.g. in a silica shell. However, no forensic analyses at the single-particle level are discussed.
The invention is based on the object of providing a security feature improved with regard to anti-forgery security, and a value document furnished with such a security feature. A further object is to provide a method for checking the authenticity of the value document.
This object is achieved by the feature combinations defined in the main claims. Preferred embodiments are the subject matter of the subclaims.